A Matter of Time

Even though evolution is taking place all around us, for many species the process operates so slowly that it is not observable except over thousands or hundreds of thousands of years — much too long to witness in a human lifetime. There are cases in quickly reproducing life forms like bacteria and fruit flies, however, where evolution can be seen happening in a matter of weeks for the bacteria and many months for the flies. In these cases the relatively large number of generations in a given period of time is key, since evolutionary change occurs incrementally from one generation to the next. All else being equal, the more generations you have, the more quickly evolution happens.

 

2. Can you observe evolution happening?

Because for many species, humans included, evolution happens over the course of many thousands of years, it is rare to observe the process in a human lifetime. Usually only laboratory scientists studying quickly reproducing life forms, like single-celled creatures and some invertebrates, have the opportunity to see evolutionary change happen before their eyes. All of us can and do experience the indirect effects of evolution nearly every day, however. One of the more important evolutionary concerns facing humans today is the emergence of antibiotic-resistant microbes. A battle against bacteria that we have been winning with medicine for the last 50 years or so is now an even race, according to some scientists — because of the rapid rate of bacterial evolution. Similarly, the use of pesticides in agriculture has driven the evolution of resistant insects that require more or harsher chemicals to be killed. Scientists studying Galapagos finches have seen evolutionary changes in beak size and shape in just a few years. Major evolutionary transformations take much, much longer.

 

3. How old is Earth?

Earth is approximately 4.55 billion years old. Radiometric dating, a method that measures the level of radioactive decay in rocks to determine how old they are has consistently aged moon rocks and meteorites at 4.4 to 4.6 billion years old. Ages of these types of rocks provide the most accurate estimates of the age of Earth and the rest of the solar system because they have not been subjected to the same forces that recycle Earth’s crust. As new land forms along cracks between the planet’s continental plates, old rocks are destroyed. Thus, the oldest rocks on Earth may not exist anymore. The oldest dated minerals, at 4.0 to 4.2 billion years old, are tiny zircon crystals found in sedimentary rocks in western Australia.

 

4. How long has life existed on Earth?

The oldest known fossils are approximately 3.5 billion years old, but some scientists have discovered chemical evidence suggesting that life may have begun even earlier, nearly 4 billion years ago.

 

5. How long ago did dinosaurs exist?

Dinosaurs existed between 230 million years ago and 65 million years ago, but none of the known dinosaur species existed for this entire time period. Throughout the group’s existence, individual dinosaur species were evolving and going extinct. Some species diverged and gave rise to other species, while others disappeared. A mass extinction event at the end of the Cretaceous period, 65 million years ago, ended the reign of dinosaurs on Earth. Recently, many scientists have come to the conclusion that, while dinosaurs may have disappeared, one dinosaur lineage had evolved into birds long before the extinction event that wiped out the other dinosaurs — and so, in a sense, dinosaurs are still around today.

 

6. How long ago did humans become human?

The oldest known hominid, or humanlike species, has been dated at 4.4 million years old. Another species, which is yet to be confirmed as a hominid, has been dated at 6 million years old. Scientists estimate that the hominid lineage diverged from the ape lineage 5 to 8 million years ago. Homo sapiens, the species to which we belong, has existed for about 100,000 years.

 

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